1. Field of the Invention
The present invention relates generally to the production of a cold-rolled stainless steel strip from a continuous-cast steel strip, and more specifically, to a method of rapidly and uniformly widthwise cooling a cast stainless steel strip having a thickness close to that of a cold-rolled stainless steel strip product when producing such a cast strip by using a synchronous type continuous caster in which there is no relative speed difference between a cast strip and an inside wall of a casting mold, particularly a vertical type twin-roll continuous caster. In this method, a cast strip having passed through a "kissing point" or a gap between a pair of cooling rolls is subsequently rapidly cooled by being kept in contact with one of the cooling rolls, to produce a fine-grained cast strip which is advantageously used as a material for producing a cold-rolled stainless steel strip having a superior surface quality.
2. Description of the Related Art
In the conventional manufacture of cold-rolled stainless steel strips by using a continuous casting process, a 100 mm or more thick steel slab is cast by using a oscillating mold, and the cast slab is then surface-finished, heated to a temperature of 1000.degree. C. or higher in a heating furnace, and hot-rolled by a hot strip mill consisting of a rough roll array and a finishing roll array to form a hot strip several mm thick.
Before cold rolling, the thus-obtained hot strip is annealed to soften the heavily hot-worked structure thereof, and surface scale, etc., is removed by pickling followed by grinding, to ensure the cold-rolled shape or flatness, the mechanical property, and the surface quality required for a final cold-rolled strip product.
The conventional process requires lengthy facilities for hot rolling and a vast amount of energy is consumed for heating and working a material, and further, this process is disadvantageous from the viewpoint of efficiency. The use of the product sheet is also limited in many respects, for example, anisotropy must be taken into consideration when working the sheet by press working, etc., because of a sheet texture which has been well-developed by many working steps during the process of forming a 100 mm or more thick cast slab into a final cold-rolled strip.
To solve the problem of the lengthy facilities for hot rolling, the vast amount of energy consumed, and the rolling power required to roll a 100 mm or more thick slab to form a hot strip, studies were recently carried out on a process in which a continuous casting provides a cast strip having a thickness substantially equivalent to or close to that of a hot strip. For example, special reports in "Tetsu-to-Hagane", 1985, pages A197 to A256 disclosed processes in which such a cast strip is directly obtained by a continuous casting. In these reports, use of a twin-roll type continuous casting is considered for obtaining a cast strip having a thickness of 1 mm to 10 mm.
These continuous casting processes, however, have problems in the casting step per se, and do not provide an effective solution to the problems of the mechanical properties and surface quality.
In these new and developing processes in which the continuous casting provides a cast strip having a thickness equivalent or close to that of a hot strip, the process from casting to a final product strip is simplified, or several process steps are omitted, with the result that the surface property of a final cold-rolled strip is easily affected by the cast strip quality. Namely, a good cast strip is required to obtain a final cold-rolled strip having a superior surface quality.
Special care must be taken to prevent a nonuniform luster and a surface defect called "roping", which are peculiar to cold-rolled stainless steel strips and commercially devalue the product sheet.
The occurrence of surface defects such as roping has a close relationship to a coarsening of the solidified structure of a cast strip. In the vertical type twin-roll continuous caster, a cast strip is separated from the cooling rolls as it leaves the kissing point between the cooling rolls, and thus is no longer rapidly cooled by a metal contact with the cooling rolls but is only air-cooled. Accordingly, a cast strip stays for a longer time at high temperatures at which the grain growth is accelerated, to cause a grain coarsening of the cast strip and surface defects of the final product strip. Therefore, it is very important to rapidly cool the cast strip which has passed through the kissing point, to prevent a coarsening of the cast structure.
To ensure the rapid cooling of a cast strip, it is most effective to keep a cast strip in contact with the cooling surface or the outer circumferential surface of a cooling roll after the cast strip has passed through the kissing point.
To this end, Japanese Examined Patent Publication (Kokoku) No. 63-19258 (hereinafter referred to as "Publication (1)") proposed a process in which a cast strip is kept in contact with the cooling roll surface by being imparted with a tensile force, and Japanese Unexamined Patent Publication (Kokai) No. 63-68248 (hereinafter referred to as "Publication (2)") proposed a process in which a plurality of auxiliary water-cooled rolls are disposed along the circumference of a main cooling roll so that a cast strip is cooled as it moves between the main and auxiliary cooling rolls.
These proposals may be effective in the case of a cast strip in the form of a ribbon and having a relatively small width, but the following basic problem arises when they are adopted in the production of a cold-rolled stainless steel strip having a large width, for which a cast strip must also have a large width.
The cooling roll of a continuous caster has flow paths for a coolant water provided inside the roll, and therefore, has a significantly lower rigidity and a greater thermal distortion in comparison with other rolls such as rolling rolls. Namely, the cooling rolls unavoidably have a structure such that both ends of the cooling roll body have a high rigidity as a support which imparts a rigidity to the whole body of a roll, but the intermediate portion of the roll body length has a lower rigidity. Accordingly, the diameter of the intermediate portion of the roll body is relatively expanded when the roll temperature is raised and relatively contracted when the roll temperature is lowered, so that the roll crown or roll curve is significantly varied by changes in the roll temperature.
As a hot, cast strip moves down and is separated from the roll surface at the kissing point, the roll temperature in the portion below the kissing point is lowered to cause a sharpening of the roll crown curvature in that portion in comparison with the roll crown curvature of the portion above the kissing point or at the portion in contact with the hot cast strip. The cast strip crown or the curvature across the strip width is determined by the roll crown of the roll portion above the kissing point and having a less sharp curvature. Accordingly, if the cast strip moving down out of the kissing point and having a less sharp crown is simply brought into contact with the roll surface situated below the kissing point and having a sharper curvature, only the side edge portions of a cast strip are actually brought into contact with the longitudinal ends of the cooling roll, and therefore, a rapid cooling over the entire width of cast strand cannot be effected.
The previously mentioned Japanese Patent Publications (1) and (2) do not take the above problem into consideration and are therefore unsatisfactory as a method of rapidly cooling a cast strip to prevent surface defects of the cold-rolled stainless steel strips. In the method of Publication (1), it is possible to increase the tensile force to an extent such that the intermediate portion of the strip width can be also brought into contact with the cooling roll surface, but in such a case, an extremely large tensile force would be loaded on the just-solidified cast strip at the kissing point, to cause the danger of, for example, a rupture of the cast strip. Thus, the method of Publication (1) cannot be practically adopted.